Rankine cycle apparatus have been known as systems for converting heat energy into mechanical work. The Rankine cycle apparatus include a structure for circulating water as a working medium, in the liquid and gaseous phases within a sealed piping system forming a circulation system in the apparatus. Generally, the Rankine cycle apparatus include a water supplying pump unit, an evaporator, an expander, a condenser, and pipes connecting between these components to provide circulation circuitry.
FIG. 19 hereof is a schematic block diagram of a general setup of a conventionally-known Rankine cycle apparatus (e.g., vehicle-mounted Rankine cycle apparatus) and certain details of a condenser employed in the Rankine cycle apparatus. The Rankine cycle apparatus of FIG. 19 includes a water supplying pump unit 110, an evaporator 111, an expander 107, and the condenser 100. These components 110, 111, 107 and 100 are connected via pipes 108 and 115, to provide circulation circuitry in the apparatus.
Water (liquid-phase working medium), which is supplied, a predetermined amount per minute, by the water supplying pump unit 110 via the pipe 115, is imparted with heat by the evaporator 111 to turn into water vapor (gaseous-phase working medium). The vapor is delivered through the next pipe 115 to the expander 107 that expands the water vapor. Mechanical device (not shown) is driven through the vapor expansion by the expander 107 so as to perform desired mechanical work.
Then, the expanded water vapor is delivered through the pipe 108 to the condenser 100, where the vapor is converted from the vapor phase back to the water phase. After that, the water is returned through the pipe 115 to the water supplying pump unit 110, from which the water is supplied again for repetition of the above actions. The evaporator 111 is constructed to receive heat from an exhaust pipe extending from the exhaust port of the engine of the vehicle. Among various literatures and documents showing structural examples of the Rankine cycle apparatus is Japanese Patent Laid-Open Publication No. 2002-115504.
The following paragraphs detail a structure and behavior of the condenser 100 in the conventional vehicle-mounted Rankine cycle apparatus, with reference to FIG. 19.
The condenser 100 includes a vapor introducing chamber 101, a water collecting chamber 102, and a multiplicity of cooling pipes 103 vertically interconnecting the two chambers 101 and 102. In the figure, only one of the cooling pipes 103 is shown in an exaggerative manner. Substantial upper half of the interior of each of the cooling pipes 103 is a vapor (gaseous-phase) portion 104, while a substantial lower half of the interior of the cooling pipe 103 is a water (liquid-phase) portion 105. In the vapor portion 104, most of the working medium introduced via the vapor introducing chamber 101 to the cooling pipe 103 is in the gaseous phase, while, in the water portion 105, most of the working medium flowing through the cooling pipe 103 is kept in the liquid (condensed water) phase. Boundary between the vapor 104 and the water 105 (i.e., gas-liquid interface) is a liquid level position 112.
One cooling fan 106 is disposed behind the cooling pipes 103 (to the right of the cooling pipes 103 in FIG. 19). The cooling fan 106 is surrounded by a cylindrical shroud 106a. Normally, operation of the cooling fan 106 is controlled by an electronic control unit on the basis of a water temperature at an outlet port of the condenser 100. The single cooling fan 106 sends air to the entire region, from top to bottom, of all of the cooling pipes 103 to simultaneously cool the cooling pipes 103.
The condenser 100 operates as follows during operation of the Rankine cycle apparatus. Water vapor of a relatively low temperature, discharged from the expander 107 with a reduced temperature and pressure, is sent into the vapor introducing chamber 101 of the condenser 100 via the low-pressure vapor pipe 108 and then directed into the cooling pipes 103. Cooling air 109 drawn into the cooling fan 106 is sent to the condenser 100.
Strong cooling air is applied by the cooling fan 106 to the upstream vapor portion 104 of the condenser 100, i.e. a portion of each of the cooling pipes 103 where a mixture of the vapor and water exists, and thus latent heat emitted when the vapor liquefies can be recovered effectively by the cooling air. Cooling air is also applied by the cooling fan 106 to the downstream water portion 105 of the condenser 100, i.e. a portion of each of the cooling pipes 103 where substantially only the water exists. Water condensed within the cooling pipes 103 of the condenser 100, is collected into the water collecting chamber 102 and then supplied by the water supplying pump unit 110 to the evaporator 111 in a pressurized condition as noted above.
In FIG. 19, reference numeral 116 represents a surface area of a condensing heat transmission portion, and 117 represents a surface area of a heat transmission portion of the condensed water. The surface areas 116 and 117 of the heat transmission portions and the liquid level position 112 have the following relationship.
The conventional Rankine cycle apparatus 100 inherently has the characteristic that the liquid fluid position 112 varies. Namely, because the engine output varies in response to traveling start/stop and transient traveling velocity variation of the vehicle, the amount of water supply to the evaporator 111 also varies, in response to which the liquid level position 112 within the condenser 100 varies. Namely, in the condenser 100, the liquid level position 112 rises when the amount of the vapor flowing into the condenser 100 (i.e., inflow amount of the vapor) is greater than the amount of the condensed water discharged from the condenser 100 (i.e., discharge amount of the condensed water), but lowers when the inflow amount of the vapor is smaller than the discharge amount of the condensed water. In this way, the vapor-occupied portion (104) in the cooling pipes 103 of the condenser 100 increases or decreases. Because the condensed water (in the portion 105) is discharged from the water supplying pump unit 110 subjected to predetermined flow rate control, a pressure from an outlet port 113 of the expander 107 to an inlet port 114 of the water supplying pump unit 110 is determined by a pressure within the condenser 100. The pressure within the condenser 100 is determined by an amount of condensing heat exchange caused by cooling of the vapor portion of the condenser, and the amount of condensing heat exchange is determined by a flow rate of the medium to be cooled and a surface area of the condensing heat transmission portion 116. Thus, if the portion occupied with the vapor increases or decreases due to variation (rise or fall) of the liquid level position 112, the surface area 116 of the condensing heat transmission portion increases or decreases and so the pressure within the condenser 100 and the flow rate of the medium to be cooled do not uniformly correspond to each other any longer.
Similarly, the temperature of the condensed water at the outlet port of the condenser 100 is determined by an amount of heat exchange caused by cooling of the water portion (105) of the condenser, and the amount of the heat exchange of the condensed water is determined by the flow rate of the medium to be cooled and a surface area 117 of a heat transmission portion of the condensed water. Thus, if the portion occupied with the condensed water (105) increases or decreases due to variation (rise or fall) of the liquid level position 112, the surface area 117 of the heat transmission of the condensed water portion increases or decreases and so the temperature of the condensed water and the flow rate of the medium to be cooled do not uniformly correspond to each other any longer. When the high-temperature vapor has reached an unusually high pressure due to some system anomaly in the above-described Rankine cycle apparatus, there arises a need to promptly restore the vapor from the unusually high pressure to a normal pressure without hindering the functions of relevant components.
For that purpose, a chlorofluorocarbon-turbine composite engine disclosed in Japanese Patent Laid-Open Publication No. SHO-49-92439 includes a pressure relief valve provided in a branch vapor pipe. Namely, in this composite engine, the outlet of an evaporator and the inlet of a condenser are connected by the branch vapor pipe via the relief valve, so that vapor can be bypassed when the interior pressure of the evaporator is at high level. However, with this composite engine, which is constructed to only adjust the pressure via the pressure relief valve provided in the branch vapor pipe, it is difficult to appropriately control a high-pressure vapor in and near the evaporator.
Further, Japanese Utility Model Laid-Open Publication No. SHO-58-124603 discloses a Rankine cycle apparatus which includes control valves between a condenser and a liquid tank and near the outlet of an evaporator. The control valves function to close circulation circuitry while the apparatus is in an OFF state or in a non-operating state, so as to prevent a liquid-phase working medium from filling an expander and condenser. With these control valves, however, the disclosed Rankine cycle apparatus can not quickly respond to a pressure increase between a water supplying pump and the evaporator.
Generally, when a high pressure, exceeding an allowable maximum pressure level of the expander or evaporator, has been produced within the circulation circuitry of the Rankine cycle apparatus, for example, due to a stagnated flow of the working medium, there arises a need to discharge the high-temperature and high-pressure working medium out of the circulation circuitry in order to promptly lower the pressure so that the expander, evaporator, etc. can be properly protected and can readily resume their operations. In such a case, it is necessary to lower the temperature and pressure of the working medium itself and minimize adverse influences exerted by the working medium on peripheral devices, such as an exhaust device of a vehicle engine.
Further, it is necessary to lower the pressure in quick response to a high-pressure vapor in and near the evaporator and a rapid pressure increase, beyond the allowable maximum pressure level, of water between the pump and the evaporator.